In a MOPA (Master Oscillator Power Amplifier)-configuration laser system in which amplifiers are in a multistage configuration, spontaneous emissions are generated between various levels and outputted, due to pumping in the amplifiers. Due to the spontaneously emitted light, an induced emission is generated and amplified, whereby rays are outputted in directions that are the same or opposite to the direction in which an originally required laser beam advances. Hereinafter, that rays are referred to as spontaneously amplified rays. It has been a problem that, when entering the adjacent amplifier, the spontaneously amplified ray reduces the gain of the amplifier, whereby the output of the originally required laser beam is reduced.
In some of communication-use laser systems, in a MOPA configuration, having a low-energy oscillator, an acoustooptic modulator is interposed between neighboring amplifiers so as to prevent the foregoing problem (e.g., refer to Patent Literature 1).
Unlike a laser system in a MOPA configuration, a laser system is utilized in which, as an amplifier, a resonator is incorporated. Among the foregoing systems in which, by making a pulse laser beam outputted from an oscillator enter an unstable resonator and amplifying the pulse laser beam in the unstable resonator, a high-power pulse laser beam is obtained, there is a high-output pulse laser system in which, in order to prevent an oscillated continuous-wave laser beam from leaking into the unstable resonator, thereby reducing the gain of the oscillator, the ON-timing of the Q switch for the amplifier is made to lag behind the ON-timing of the Q-switch for the oscillator (e.g., refer to Patent Literature 2).    [Patent Literature 1] Japanese Laid-Open Patent Publication No. 1998-268369    [Patent Literature 2] Japanese Laid-Open Patent Publication No. 2001-308427